Switch valves of this kind are distinguished by a particularly simple design since they are moved by control only between two switching states, specifically fully open or fully closed. Therefore, the switch valve does not assume any intermediate positions.
The switch valves are actuated by means of a pulse-width-modulated (PWM) signal. This signal is defined by the control device on the basis of various setpoint values of corresponding operating parameters. However, there is a risk in this case of deviations in the corresponding actual values from the setpoint values, for example due to component tolerances, operating tolerances etc., giving rise to faults which cause the opening time of the switch valve to be incorrectly determined. When, for example, the aim is to achieve a desired fluid throughflow rate using the switch valve, a value which differs from said desired fluid throughflow rate results in reality.
These problems will be explained using a conventional tank venting system. A system of this kind has an activated carbon filter which binds hydrocarbons which are outgassed from the tank. A flushing line leads from the activated carbon filter, via a tank venting valve, into the intake pipe of the associated engine. The tank venting valve is an above-described switch valve which is driven by the engine controller by means of a pulse-width-modulated (PWM) signal and controls the flow of gas from the activated carbon filter to the intake pipe. This valve is actuated, for example, at a frequency of between 5 and 35 Hz.
The associated engine controller determines, amongst other things, a setpoint value for the flushing flow, and furthermore the intake pipe pressure with the aid of a pressure sensor, for the current operating state. A PWM value for actuating the tank venting valve is determined from the pressure gradient between ambient pressure and intake pipe pressure and from the prespecified flushing flow. The abovementioned distortions can also occur in this case.
For the purpose of eliminating the abovementioned problems, it is known to adjust the opening point in order to thereby match the opening point of the switch valve to the actually existing conditions. Adjusting the opening point in this way has been carried out in the above-described tank venting system in the case of a fully loaded activated carbon filter by interpreting the lambda deviation by slowly opening the switch valve during a stationary operating phase of the engine. Disadvantages of this method are:                1. said method is possible only when the activated carbon filter is full,        2. said method is possible only during relatively long stationary operating phases of the engine,        3. there are no other disturbing influences on the lambda controller.        
Furthermore, it is known to evaluate the corresponding current signal in order to carry out opening point detection in switch valves. However, additional costs for a current sensor are incurred in this case.